The rapid diffusion of Wireless Local Area Network (WLAN) access and the increasing demand for WLAN coverage is driving the installation of a very large number of Access Points (AP). The most common WLAN technology is described in the Institute of Electrical and Electronics Engineers IEEE 802.11 family of industry specifications, such as specifications for IEEE 802.11b, IEEE 802.11g and IEEE 802.11a. Other wireless technologies are being developed, such as IEEE 802.16 or WiMAX technology. A number of different 802.11 task groups are involved in developing specifications relating to improvements to the existing 802.11 technology. A draft specification from the IEEE 802.11e Task Group has proposed a set of QoS parameters to be used for traffic between an Access Point and a station. See, e.g., Tim Godfrey, “Inside 802.11e: Making QoS A Reality Over WLAN Connections,” CommsDesign, Dec. 19, 2003. These are a few examples of wireless network specifications or proposed standards, and there are many other wireless technologies and standards being developed.
A number of wireless standards allow stations to access a channel through a contention based channel access mechanism, where wireless nodes may contend for channel access (or access to the wireless medium). For example, in WiMedia Distributed MAC specification, this channel access technique is referred to as prioritized contention access (PCA), where contention access is provided using different access categories (ACs), or traffic priorities. For example, for WiMedia and for 802.11e Enhanced Data Channel Access (EDCA), different quality of service (QoS) parameters may be provided for each AC. For example, QoS parameters may include: CWmin[AC], which is the minimum contention window for the AC, the CWmax[AC], the AIFSN[AC] which is the arbitration inter-frame spacing for the AC, the TXOPLimit[AC] which defines the length of the transmission opportunity or TXOP a wireless node is granted. Values for these parameters may be established to favor higher ACs or over lower ACs, for example.
In some wireless networks (such as 802.11 Wireless LAN networks as an example), virtual carrier-sense and physical carrier-sense functions may be used to determine the availability of the shared medium or channel. For example, the medium may be considered to be idle in some cases when both of these two functions indicate that the medium is idle. While the physical carrier-sense function uses the physical layer to sense the carrier, the virtual carrier-sense function is based on the Network Allocation Vector (NAV). For example, most IEEE 802.11 frames may carry a duration field, which may be used to reserve the medium (or channel) for a fixed time period. The NAV may be considered a timer that indicates the amount of time for which the medium has been reserved. Transmitting nodes may typically set the NAV (using the duration field in frames) to the time for which they expect to use the medium, including the transmission time of all the frames in a sequence. Other nodes in the network may set up a process to count down the NAV (a locally stored copy of the NAV information). When the NAV is greater than zero, the virtual carrier-sense function indicates that the medium is busy. When the NAV reaches zero, the medium is reported to be idle.
In addition, wireless nodes may use a Request-to-Send (RTS) and Clear-to-Send (CTS) exchange to request permission or clearance to use the medium or channel. A requesting node may typically send a RTS, and then receive a CTS. The CTS may indicate that the channel or medium is idle or available and may serve to warn other nodes that the medium is now busy, since the RTS and CTS frames also typically include a duration field. The use of RTS/CTS may provide increased protection for a frame since a RTS/CTS exchange may decrease the likelihood of a collision with another frame. The NAV mechanism and/or the RTS/CTS exchange may be used in wireless networks to reduce frame collisions and to reduce “hidden node” problems.
To limit overhead, RTS/CTS protection is typically used only for frames that exceed a size threshold. This is a fairly simple approach, and there may be other factors or criteria that may be useful in determining when to use the RTS/CTS protection.